It has been conducted that by making a pressure-sensitive adhesive layer of pressure-sensitive adhesive tapes or sheets be a fine foamed structure, an apparent modulus of elasticity is decreased to improve properties such as an initial pressure-sensitive adhesive property, adhesive property to a rough surface or repulsion resistance, and the amount of a pressure-sensitive adhesive used is decreased to reduce a material cost.
Conventional methods for producing the above-described foamed pressure-sensitive adhesive tape or sheet include a method of incorporating a thermal decomposition type chemical foaming agent into a pressure-sensitive adhesive layer followed by heating as disclosed in, for example, JP-A-62-263278) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and a method of intermixing fine hollow particles such as microcapsules in a pressure-sensitive adhesive layer as disclosed in, for example, JP-A-2-240182. However, those methods have involved various problems such that the foaming agent or fine hollow particles are difficult to uniformly disperse, fine cells are difficult to obtain, it is difficult to control the foamed cell structure, and a cost increases.
JP-A-8-199125 discloses a method foaming the pressure-sensitive adhesive layer by placing an existing pressure-sensitive adhesive sheet under a high pressure gas atmosphere to dissolve the atmospheric gas therein and then releasing the pressure. However, this method can form a fine cell structure, but the conventional production steps of a pressure-sensitive adhesive sheet, such as coating and drying the pressure-sensitive adhesive, remain unchanged, and in addition to this, a foaming step is necessary. As a result, the steps become complicated, and the cost is unavoidably increased as compared with the production cost of the conventional pressure-sensitive adhesive sheet. Also, since the foaming step is mainly carried out in a high-pressure room of a batch system, this method is very disadvantageous from the standpoint of a continuous productivity.